Modification Of XLi₂MnO₃·（1-x）LiMO₂（M=Ni,Mn,Co） For Lithium-ion Batteries

With the progress of technology and social development, requirement for energystorage equipment are increasingly demanded in many fields, such as the electronicproducts, medical equipment, clean energy vehicles and high-tech weapons. Developinglow-cost, high-safety, high-capacity and high-power electrode materials has become thetrend in lithium ion batteries community. The layered lithium-rich oxide materials,xLi2MnO3·(1–x)LiMO2(M=Mn, Ni, and Co), are becoming one of the most promisingcathode materials recently, because of their high capacities of～250mAh·g-1andsignificantly reduced cost. However, there are some drawbacks of these cathode materials,such as low initial coulombic efficiency, intrinsic poor rate capability and so on. In thisthesis, the layered lithium-rich oxide material Li[Li0.2Mn0.55Ni0.15Co0.1]O2, has beensynthesized by a sol–gel method, and then were coated with Li4Mn5O12and Li4Ti5O12respectively, so as to improve the electrochemical performance of the materials. Thespecific work results are summarized as follows:We prepared Li[Li0.2Mn0.55Ni0.15Co0.1]O2by a sol–gel method, and an attempt havebeen made to coated the pristine materials with high Li+conductive Li4Mn5O12. The X-raydiffraction studies show that the main structure of modified materials has been hardlychanged, preserving the layered structure with well crystallinity. Notably, weak shoulderpeaks appear with increment of coating amount, which is ascribed to the generation ofLi4Mn5O12spinel phase. The electrochemical performance tests show that0.01Li4Mn5O12·0.99Li[Li0.2Mn0.55Ni0.15Co0.1]O2exhibit the best cycle stability (thediacharge capacity attains259.8mAh g-1ater80cycles at0.1C, with a capacity retention of89.8%) and rate performance(the diacharge capacity attains185.1mAh g-1ater50cycles at1C and the initial discharge capacity is138.8mAhg-1at5C). Moreover, the charge transferimpedance of the material is significantly reduced after coated with Li4Mn5O12. The TEMresults show that the0.01Li4Mn5O12·0.99Li[Li0.2Mn0.55Ni0.15Co0.1]O2materials has a thincoating layer of Li4Mn5O12with a width of2-3nm, which can effectively protect the bulkparticles from the erosion of electrolytes and restrain the dissolution of transition metal ions,thus improving cycle stability and rate performance. We prepared Li[Li0.2Mn0.55Ni0.15Co0.1]O2by a sol–gel method as discussed before, anda way have been tried to surface modify pristine materials with Li4Ti5O12of high stablestructure and high Li+conductive. The X-ray diffraction studies show that the mainstructure of modified materials has been hardly changed, preserving the layered structurewith well crystallinity. Notably, weak shoulder peaks appear with increment of coatingamount, which is related with the existence of Li4Ti5O12spinel phase. The electrochemicalperformance tests show that0.03Li4Ti5O12·0.97Li[Li0.2Mn0.55Ni0.15Co0.1]O2exhibit the bestcycle stability (the diacharge capacity attains200.2mAh g-1mAh g-1ater50cycles at0.1C,with a capacity retention of80.5%) and rate performance(the diacharge capacity attains181.3mAh g-1at1C and the initial discharge capacity is156.1mAhg-1at5C). Moreover, thecharge transfer impedance of the material is relatively reduced after surface modified withamounts of Li4Ti5O12. The TEM results show that the0.03Li4Ti5O12·0.97Li[Li0.2Mn0.55Ni0.15Co0.1]O2materials has a thin Li4Ti5O12layer. This thin layer Li4Ti5O12with a high structural stability, can effectively improve the structural stability of the bulklayered materials.